This invention relates to a high-pressure mercury-vapor lamp with a phosphor coating.
High-pressure mercury-vapor lamps are most popular as the lamps for outdoor illumination, and are expected to be widely used in the future because of their stable lamp characteristics and excellent economic advantages. These lamps may be roughly divided into the transparent type and the diffusion type. In the former, only the visible line spectrum of mercury generated from the arc discharge is utilized for illumination and accordingly, its color rendition index Ra runs as low as about 25; whereas in the latter, the ultraviolet radiations of mercury are utilized to stimulate the phosphors to cause them to emit visible rays by coating the inside surface of the outer bulb with a phosphor layer, to substantially improve its color rendering property. For example, these color corrected high pressure mercury vapor lamps in which such red light emitting phosphors as yttrium vanaphosphate or yttrium vanadate activated with trivalent europium are used, may provide a general color rendering index of about 42-45. Even these color corrected high pressure mercury vapor lamps having improved color rendition are still unsatisfactory for lamps for general indoor illuminations.
Lamps of high brightness for indoor illumination of offices and lobbies have recently come into great demand. High-pressure mercury-vapor lamps available on the market have failed to meet this demand, because of their inferior color rendering property.